This research proposal addresses the molecular mechanism by which two T -box transcription factors, spadetail (spt) and no tail (ntl), selectively regulate the formation of the trunk and tail in the zebrafish embryo. This will be addressed by analyzing the regulation of another T-box factor, thx6, during the regional development of the posterior body. The objective of this work is to examine the hypothesis that spt and ntl serve as "selector genes" to regulate the trunk and tail expression of thx6, via the selective cooperation of additional coregulators. The specific aims are: 1) To determine the principle role of thx6 in the formation of trunk and tail mesoderm, 2) To examine the ability of spt and ntl to directly activate thx6 expression, 3) To investigate the role of Bmps in the regulation of thx6 expression, and 4) To identify spt and ntl dependent regulatory elements within the thx6 gene. Determining the regulatory network activated by spt and ntl will provide new insights into our current understanding of transcriptional mechanisms that subdivide the development of the vertebrate body.